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Sets and Probability

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Sets and Probability CHAPTER 1 Sets and Probability In a survey of 200 people that had just returned from a trip to Europe, the following informa- tion was gathered. • 142 visited England • 95 visited Italy • 65 visited Germany • 70 visited both England and Italy • 50 visited both England and Germany • 30 visited both Italy and Germany • 20 visited all three of these countries How many went to England but not Italy or Germany? We will learn how to solve puzzles like this in the second section of the chapter when count- ing the elements in a set is discussed. Exerpt from Applied Finite Mathematics by Tomastik and Epstein (c) 2008 Cengage Learning 1.1 Introduction to Sets 13 1.1 Introduction to Sets This section discusses operations on sets and the laws governing these set opera- tions. These are fundamental notions that will be used throughout the remainder of this text. In the next two chapters we will see that probability and statistics are based on counting the elements in sets and manipulating set operations. Thus we first need to understand clearly the notion of sets and their operations. ✧ The Language of Sets We begin here with some definitions of the language and notation used when working with sets. The most basic definition is “What is a set?” A set is a HISTORICAL NOTE collection of items. These items are referred to as the elements or members of George Boole, 1815–1864 the set. For example, the set containing the numbers 1, 2, and 3 would be written {1,2,3}. Notice that the set is contained in curly brackets. This will help us George Boole was born into a lower-class family in Lincoln, distinguish sets from other mathematical objects. England, and had only a common When all the elements of the set are written out, we refer to this as roster school education. He was largely notation. So the set containing the first 10 letters in the English alphabet would self-taught and managed to { , , , , , , , , , } become an elementary school be written as a b c d e f g h i j in roster notation. If we wanted to refer to teacher. Up to this time any rule of this set without writing all the elements, we could define the set in terms of its algebra such as a(x + y)=ax + ay properties. This is called set-builder notation. So we write was understood to apply only to numbers and magnitudes. Boole { | } developed an “algebra” of sets x x is one of the first 10 letters in the English alphabet where the elements of the sets could be not just numbers but This is read “the set of all x such that x is one of the first 10 letters in the English anything. This then laid down the alphabet”. If we will be using a set more than once in a discussion, it is useful to foundations for a fundamental way of thinking. Bertrand Russell, a define the set with a symbol, usually an uppercase letter. So great mathematician and philosopher of the 20th century, S = {a,b,c,d,e, f ,g,h,i, j} said that the greatest discovery of the 19th century was the nature of { , , , , , , , , , } pure mathematics, which he We can say c is an element of the set a b c d e f g h i j or simply write asserted was discovered by c ∈ S. The symbol ∈ is read “is an element of”. We can also say that the set George Boole. Boole’s pamphlet R = {c} is a subset of our larger set S as every element in the set R is also in the “The Mathematical Analysis of Logic” maintained that the set S. essential character of mathematics lies in its form rather than in its content. Thus mathematics is not Subsets merely the science of If every element of a set A is also an element of another set B, we say that measurement and number but any ⊆ study consisting of symbols and A is a subset of B and write A B.IfA is not a subset of B, we write precise rules of operation. Boole A ⊆ B. founded not only a new algebra of sets but also a formal logic that we will discuss in Chapter L. Thus {1,2,4}⊆{1,2,3,4},but{1,2,3,4} ⊆{1,2,4}. Since every element in A is in A, we can write A ⊆ A. If there is a set B and every element in the set B is also in the set A but B = A, we say that B is a proper subset of A. This is written as B ⊂ A. Note the proper subset symbol ⊂ is lacking the small horizontal line that the subset symbol ⊆ has. The difference is rather like the difference between < and ≤. Some sets have no elements at all. We need some notation for this, simply leaving a blank space will not do! Exerpt from Applied Finite Mathematics by Tomastik and Epstein (c) 2008 Cengage Learning 14 Chapter 1 Sets and Probability Empty Set The empty set, written as0or / {}, is the set with no elements. The empty set can be used to conveniently indicate that an equation has no solution. For example {x|x is real and x2 = −1} = /0 By the definition of subset, given any set A, we must have /0 ⊆ A. EXAMPLE 1 Finding Subsets Find all the subsets of {a,b,c}. Solution The subsets are /0,{a},{b},{c},{a,b},{a,c},{b,c},{a,b,c} ✦ REMARK: Note that there are 8 subsets and 7 of them are proper subsets. In general, a set with n elements will have 2n subsets. In the next chapter we will learn why this is so. The empty set is the set with no elements. At the other extreme is the uni- versal set. This set is the set of all elements being considered and is denoted by U. If, for example, we are to take a national survey of voter satisfaction with the president, the universal set is the set of all voters in this country. If the survey is to determine the effects of smoking on pregnant women, the universal set is the set of all pregnant women. The context of the problem under discussion will determine the universal set for that problem. The universal set must contain every element under discussion. A Venn diagram is a way of visualizing sets. The universal set is represented by a rectangle and sets are represented as circles inside the universal set. For Figure 1.1 example, given a universal set U and a set A, Figure 1.1 is a Venn diagram that visualizes the concept that A ⊂ U. Figure 1.1 also visualizes the concept B ⊂ A. The U above the rectangle will be dropped in later diagrams as we will abide by the convention that the rectangle always represents the universal set. ✧ Set Operations The first set operation we consider is the complement. The complement of set A are those members of set U that do not belong to A. Complement Given a universal set U and a set A ⊂ U, the complement of A, written Ac, is the set of all elements that are in U but not in A, that is, Ac = {x|x ∈ U, x ∈/ A} Figure 1.2 Ac is shaded. Exerpt from Applied Finite Mathematics by Tomastik and Epstein (c) 2008 Cengage Learning 1.1 Introduction to Sets 15 A Venn diagram visualizing Ac is shown in Figure 1.2. Some alternate nota- tions for the complement of a set are A and A¯. EXAMPLE 2 The Complements of Sets Let U = {1,2,3,4,5,6,7,8,9}, A = {1,3,5,7,9}, B = {1,2,3,4,5}. Find Ac, Bc, Uc,/0c, and (Ac)c in roster notation. Solution We have Ac = {2,4,6,8} Bc = {6,7,8,9} Uc = /0 /0c = {1,2,3,4,5,6,7,8,9} = U (Ac)c = {2,4,6,8}c = {1,3,5,7,9} = A ✦ Note that in the example above we found Uc = /0and /0c = U. Additionally (Ac)c = A. This can be seen using the Venn diagram in Figure 1.2, since the complement of Ac is all elements in U but not in Ac which is the set A. These three rules are called the Complement Rules. Complement Rules If U is a universal set, we must always have Uc = /0, /0c = U If A is any subset of a universal set U, then (Ac)c = A The next set operation is the union of two sets. This set includes the members of both sets A and B. That is, if an element belongs to set A or set B then it belongs to the union of A and B. Set Union The union of two sets A and B, written A∪B, is the set of all elements that belong to A,ortoB, or to both. Thus A ∪ B = {x|x ∈ A or x ∈ B or both} REMARK: This usage of the word “or” is the same as in logic. It is the inclusive “or” where the elements that belong to both sets are part of the union. In English the use of “or” is often the exclusive “or”. That is, if a meal you order at a restaurant comes with a dessert and you are offered cake or pie, you really only get one of the desserts.
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